A four-stroke internal-combustion engine generally comprises a multiplicity of cylinders including a combustion chamber in which combustion of a mixture of fluids, generally air, and of a fuel takes place. The result of this combustion generates burnt gas or exhaust gas which is discharged from these chambers.
The torque delivered by such an engine notably depends on the amount of air fed into the combustion chamber of this engine, this amount of air being itself proportional to the density of this air. Thus, if a high torque is required for this engine, the air is compressed before it enters the combustion chamber. This air, generally referred to as supercharged air, is compressed by any known means such as a turbocompressor or a driven compressor such as a screw compressor for example.
If a turbocompressor is used, part of the energy lost in the exhaust gas is recovered by means of a turbine placed in the exhaust gas stream. This energy is used positively to compress the intake air, which increases air filling and therefore engine performances. Usually, this exhaust gas comes from one or more exhaust manifolds connected to the exhaust means of the cylinders.
As it is well-known in the art, the flow rate of the exhaust gas coming from the combustion chambers varies quite significantly between low-speed and high-speed running of the engine.
For low engine speeds, it is necessary to use specific exhaust manifolds so as to best recover the energy of the exhaust gas in order to drive the turbine. This is generally achieved using an exhaust manifold with small sections and volumes so as to best guide the exhaust pressure waves and to recover as much energy as possible to drive the turbine.
For high engine speeds, the small sections and volumes of such manifolds penalize emptying of the cylinders and increase pumping losses. Discharge of the burnt gas is thus slowed down and part of the gas is not discharged from the combustion chamber of the engine. This residual burnt gas generates engine knock for spark-ignition internal-combustion engines because of the high temperature thereof and furthermore limits fresh air filling of the combustion chamber during the intake phase of the engine.
The present invention aims to overcome the aforementioned drawbacks by means of an engine comprising a burnt gas discharge device of simple design, applicable to all engine speeds while absorbing the volume variations of the exhaust gas without penalizing the engine performances.